ELASTICITY 207 



stress and the amount of shear strain. It is denoted by the 



shear stress 

 letter N (sometimes by C or G) = ^^ear strain' 



Bulk Modulus is the ratio involved when, e.g., a red blood 

 corpuscle is immersed in a fluid pressing equally on it from all 

 directions. That is, when three mutually perpendicular and equal 

 direct stresses act on a body they produce a volumetric strain. 

 The volume may be increased or decreased as the result. The 

 bulk modulus is usually expressed as K. If the intensities of the 

 stresses are each = p, then 



p . . change in volume 



— = volumetric strain — r-; — -. -. . 



K origmal volume 



The volumetric strain is three times the accompanying linear 

 strain under these conditions. 



Poisson's Ratio expresses the relation between lateral and 

 longitudinal strain. A cube of rubber, if pressed on opposite 

 sides between the finger and thumb, bulges on the other four sides, 

 or, conversely, if two of the parallel sides are pulled apart, the other 

 sides become concave. That is, direct stress produces a strain in 

 its own direction, and an opposite kind of strain in every direction 

 perpendicular to its own. These two opposing strains resolve into 

 diagonals, and if the body stressed ruptures it will do so along a 

 diagonal line. This is of some importance in explaining the 

 oblique course taken by fractures of bones due to indirect violence. 



i lateral strain 



Poisson's ratio = — = -. .^ ,. — -. — 7 — —. 



m longitudmal strain 



The value of m usually lies between 3 and 4. 



Elasticity. All the tissues of the body are more or less elastic. 

 This property includes {a) change of form under the action of some 

 force and (6) the return of the body to its original form when the 

 deforming force ceases to act. 



The elasticity of connective tissues plays an important part in 

 the body. (1) It is a permanent resistance to permanent distorting 

 forces such as muscular tension and gravity. The elasticity of 

 the intervertebral discs, and of the ligamenta subflava, assists in 

 maintaining the erect posture of the body. (2) The form of 

 tissues is preserved against the distortion due to temporary forces 

 and intermittent forces, e.g., the elasticity of the costal cartilages 

 and of the ribs restores the chest wall to its original position when 

 the inspiratory muscles relax. (3) Intermittent movement is 

 transformed into a continuous movement by transmission through 

 an elastic medium (see circulation). (4) Elasticity economises 



